Comparison between three-point and four-point flexural tests to determine wood strength of Eucalyptus specimens

Paulo Ricardo Gherardi Hein,Loïc Brancheriau

doi:10.4067/s0718-221x2018005003401

Paulo Ricardo Gherardi Hein, Loïc Brancheriau

Open Access

https://doi.org/10.4067/s0718-221x2018005003401

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Abstract

Comparison between flexural wood strength values obtained from three-point and four-point bending tests in wood were only reported in few previous works, but remains unclear. Thus, the aim of this study was to determine the relationship between wood strength (MOR) obtained by 4-point and 3-point bending tests in Eucalyptus. Two or three specimens were cut from the same scantling (thus considered as twin) and submitted to flexural vibration tests. Then 190 wood specimens were tested by 4-point bending tests and 138 twin specimens were submitted to 3-point bending tests. Wood strength determined from 3-point bending test presented significantly higher mean value (76.8 MPa) than that from 4-point bending test (73.0 MPa). A linear regression for converting MOR from 3-point to 4-point bending test was proposed for these Eucalyptus specimens: MOR4p = 0.889 × MOR3p + 5.14 in MPa (R²=0.74). The correlation of MOR in 3-point bending with density was higher (R²=0.57) than in 4-point bending (R²=0.45). The correlations between dynamic elastic moduli and moduli of rupture were similar (circa R²=0.57 for MOR3p and R²=0.65 for MOR4p).

Highlights

Mechanical properties of the materials can be obtained from destructive and non destructive testing
Destructive tests are based on the application of a force on the specimen until it fails: the stress to strain plot, the maximum force until failure and the distance displaced are recorded (Kollmann and Côté 1968).According to Mujika (2006) mechanical properties, such as strength and stiffness, can be calculated using a range of test methods
The set of 328 wood specimens of Eucalyptus investigated in this study presented a wide range of values for elastic and rupture modulus (CV=~20%) with air-dried density (ρ) values ranging from ~360 to 710 kg.m-3

Summary

Introduction

Mechanical properties of the materials can be obtained from destructive and non destructive testing. Destructive tests are based on the application of a force on the specimen until it fails: the stress to strain plot, the maximum force until failure and the distance displaced are recorded (Kollmann and Côté 1968).According to Mujika (2006) mechanical properties, such as strength and stiffness, can be calculated using a range of test methods. In most cases 3-point bend and 4-point bend are suggested for prediction of flexural properties of the materials (Kumar and Murthy 2012). These bending tests simulate tensile and compression stress on a wood specimen and its physical response behavior is monitored. According to Chitchumnong et al (1989) the fundamental differences are the location of the maximum bending moment and maximum axial fiber stresses: the maximum stress occurs directly below the loading nose in three-point loading, but is spread out over the area between the loading noses in the four-point system

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